Lead dioxide, also known as anhydrous plumbic acid, lead peroxide and lead superoxide, is a very useful material. When provided in the form of a relatively thin coating upon the surface of an electrically conductive supporting base or substrate, it is useful as an electrode in corrosive electrolytes. For instance, a substrate coated with lead dioxide is useful as an anode in the electrolytic production of a wide variety of compounds exemplified by sodium chlorate; sodium perchlorate; alkali earth hypochlorites, chlorates and perchlorates; chlorine; sodium hypochlorite; sodium bromate, sodium iodate; sodium periodate; potassium chlorate and perchlorate; iodic acid; periodic acid; potassium bromate; potassium iodate; and potassium periodate. Such anodes also find applications in chrome plating.
The electrodes are also useful in carrying out a large number of electro-oxidation reactions including destruction of phenol, cyanide, and ammonia in waste water; selective oxidation of organic compounds; on-site generation of disinfectants; preparation of oxidative solutions for odor control; and oxidation and extraction of various minerals from ores.
When it is desired to produce an anode comprising a coating of lead dioxide on a base substrate for use in an electrochemical cell, it is of great importance that the lead dioxide coating be tightly adherent to the substrate material, and that the coating be uniform, nonporous, and able to withstand a substantial amount of abuse. The various prior art processes have involved the electrodeposition of lead dioxide from a solution containing a lead salt. For the most part, these processes have developed anodes which are not suitable for electrochemical use as inert or insoluble anodes because they suffered from one or more of the following defects:
(A) THE LEAD DIOXIDE DEPOSIT WAS NOT UNIFORM AND NOT SUFFICIENTLY ADHERENT TO THE ELECTRODE SURFACE;
(B) THE LEAD DIOXIDE COATING WAS TOO POROUS AND TOO COARSE;
(C) THE LEAD DIOXIDE DEPOSIT WOULD NOT WITHSTAND THE NORMAL ABUSE ASSOCIATED WITH ROUTINE HANDLING IN A COMMERCIAL PLATE; AND
(D) THE ACTIVE LIFE OF THE ELECTRODE WAS DRASTICALLY SHORTENED DUE TO EXCESSIVE PLATE CORROSION, PARTICULARLY ABOVE THE SOLUTION LEVEL AT THE ELECTRICAL BUSS CONNECTION.
These defects, the consequent unreliability of the lead dioxide anodes, and the costliness of the various expedients that have been proposed with the object of avoiding such defects, hampered their acceptance and compelled the use of platinum anodes. The latter, although more dependable, involve a very large original investment, suffer from platinum losses in processing, require a higher power input, and exhibit a lower efficiency than the anodes of this invention.
It was not until the advent of the process disclosed and claimed in U.S. Pat. No. 2,945,791, issued July 1960 to Fred. D. Gibson, Jr., and assigned to the assignee of the present application, that the manufacture and use of lead dioxide anodes became entirely practical. That patent discloses a process for the electrodeposition of lead dioxide on a graphite substrate, and it is pointed out therein how the resultant lead dioxide coating is characterized by compactness and high density, hardness, smoothness, and a fine, randomly-oriented crystalline structure firmly bound to the graphite base. These characterizing features make it possible to use the resulting anodes in the electrolytic production of chlorine, chlorates, perchlorates, and other products without any further treatment. The completed anodes incorporate the mechanical strength of the graphite substrate, thereby overcoming many of the most trying difficulties which had previously been experienced. The making of an electrical connection to the anode is extremely simple, since contact can be made directly to the uncoated upper few inches of the graphite itself and need not be made to the lead dioxide coating. This process of making lead dioxide anodes and also the superior anodes resulting from such process have enjoyed considerable commercial success, being quite widely used in various parts of the world.
Experience in using anodes prepared according to the process of U.S. Pat. No. 2,945,791 demonstrated that, although such anodes are superior to known lead dioxide anodes of the art prior thereto, anodes were nevertheless occasionally produced which did not have a coating which was tightly adherent over the entire surface. When this occurs, the corrosive electrolyte can attack the substrate during subsequent use, thereby progressively deteriorating the quality of the lead dioxide coating in that area. When such a condition is found to exist, it must of course be quickly remedied, and this requires the shutting down of the cell employing the defective anode and the replacement of such anode with a new one. This involves not only the cost of providing a new anode but also results in a decrease in production of the shutdown time of the cell. Accordingly, efforts were undertaken to increase anode life and thereby substantially reduce operating costs.
The electrolytes proposed in U.S. Pat. No. 2,945,791 and some of the art prior thereto, for deposition of lead dioxide generally included lead nitrate, copper nitrate, nickel nitrate, and nitric acid. U.S. Pat. No. 2,945,791 further suggested the incorporation in the electrolyte of small amounts of sodium fluoride and a surface-active agent. These various ingredients served various purposes. For instance, it was considered important for the copper nitrate to be present to effect preferential plating of copper rather than lead on the cathodes of the cells in which the lead dioxide anodes were being prepared. While the copper does tend to build up deposits which would eventually short-circuit the cells unless they were periodically shut down to remove such deposits, such shut-downs, though expensive, are still less objectionable than the consequences of permitting lead to deposit on the cathodes. The presence of the nickel nitrate was considered an important factor in the attainment of desirable fineness in the crystalline structure of the lead dioxide deposit. Experience in the manufacture of high purity lead dioxide, such as is required in the anodes intended for use in electrochemical production of chlorine, chlorates, perchlorates and other compounds, has shown that the above-described electrolytes should have an acid concentration of less than about 8 grams per liter. Although lead dioxide will, to be sure, plate out in the presence of higher concentrations of acid, the resultant deposit generally has been decidedly inferior in quality. In actual practice, therefore, the acid concentration was limited to about 4 to 6 grams acid per liter of electrolyte.
The continued research to produce improved lead dioxide anodes on the one hand and to reduce the cost of their production on the other hand led to the improved process disclosed in U.S. Pat. No. 3,463,707 and assigned to the assignee of the present application. The process of U.S. Pat. No. 3,463,707 made possible the production of improved electrodes while at the same time making possible the use of an electrolyte having fewer ingredients than theretofore thought necessary. Another benefit disclosed in U.S. Pat. No. 3,463,707 was the reduction in operating costs, since the more expensive ingredients of prior art electrolytes such as copper nitrate and nickel nitrate are not required in the process disclosed in U.S. Pat. No. 3,463,707. The improved results obtained by the process of U.S. Pat. No. 3,463,707 stem particularly from the use of an improved electrolyte. Broadly speaking the electrolyte of the invention of said U.S. Pat. No. 3,463,707 is one in which the acid concentration is maintained substantially above previously acceptable values in conjunction with limiting of the concentration of certain other constituents of the electrolyte to prescribed levels.
Although anodes produced according to the processes of U.S. Pat. Nos. 2,945,791 and 3,463,707 are superior to other lead dioxide anodes, occasionally anodes are produced which have imperfections in the lead dioxide coating and may not be as smooth as desired.
Accordingly, it is an object of the present invention to make possible the manufacture of lead dioxide anodes which are freer of imperfections as compared to prior art anodes.
In addition, since lead dioxide anodes lose lead by surface wear during electrolytic action during use, the ability to decrease surface wear would be a significant advantage. In particular, the loss of lead has been a problem when the anodes are used in cold electrolytes such as at temperatures of below about 15.degree. C. Therefore, it is another object of the present invention to make possible the manufacture of lead dioxide anodes having improved resistance to surface attack during electrolytic action, and particularly in relatively cold electrolytes.
Another object of the present invention is to provide lead dioxide anodes which exhibit improved current efficiency in electrolytic processes.
It is a further object of this invention to provide for the production of lead dioxide anodes which are suitable for use as an insoluble anode in electrolytic processes using corrosive electrolytes and, in particular, for the production of sodium chlorate; sodium perchlorate; chlorine; alkali earth hypochlorites, chlorates, and perchlorates; sodium hypochlorite; sodium bromate, sodium iodate; sodium periodate; potassium chlorate and perchlorate; iodic acid; periodic acid; potassium salts of bromates, iodates, and periodates, and as an inert anode in chrome plating solutions. Also, it is an object of the present invention to provide anodes which are suitable for use in electrolyzing sea water to provide hypochlorite.
Another object of the present invention is to provide a process for the production of bipolar anodes comprising flat sheets of substrate which are plated on only one side with a tightly adherent, smooth, nonporous coating containing lead dioxide.